The following is an excerpt from "The Codex Phantasma" by renowned paranormal investigator Johnathan T. Oleander. 

"Greetings, dear reader! I would like to take this time to personally thank you for purchasing this copy of my book! If you're reading this, it means you and I have something in common: a thirst for knowledge regarding the nature of death and the afterlife! Either that, or you've purchased this in hopes it contained steps to perform and exorcism to rid your home of evil entities, in which case I direct your attention to chapter 23, page 237 titled "demonic infestation and cleansing rites." Good luck and god help you. Assuming that you are NOT currently engaged in the act of fending off unknowable horrors from the darkest pits of hell, then I salute your bravery! Not many have the guts to be paranormal investigators. Our line of work is a dangerous one, and not just because of the ancient and powerful spirts you may encounter. We face constant reticule from the scientific community, who view individuals like us as "madmen" or "con artists" or simply "liars". You will have to learn to ignore these skeptics, for they are ignorant to the importance of our work. We seek to better understand the nature of life and death though the study of those who cannot or will not pass on. Only through the proper study and documentation of such beings will the mockery end. With the help of the information I have provided below, perhaps YOU will be the one to finally prove to the world that we were RIGHT." 

Supernatural Vs. Preternatural

"The first order of business is learning the different between supernatural entities and preternatural entities. A supernatural entity is the restless spirit of a deceased human. For whatever reason, this spirit's soul still clings to the living world. It could be that it seeks revenge for it's wrongful demise, it could be that it never confessed it's feelings for a love interest, it could ever be that it simple remains because it ENJOYS the terror and despair it causes. Whatever the case, it will not pass on until it believes it's goals have been achieved. On the other end of the spectrum are preternatural entities, which are nonhuman spirits that have been drawn to our world by way of summoning or the presence of strong negative emotion, which many preternatural beings feed upon."  

Danger Levels

Harmless: Cannot hurt you in any way.

Low: Can hurt you, but usually won't unless provoked.

Medium: Might attack you, but usually won't kill you on purpose.

High: WILL attack you and will likely kill you if you aren't careful.

Lethal: Will make you WISH it would kill you.

Ghost Species 

Wisps

Type: Supernatural

Threat Level: Harmless

"Wisps (AKA Ghost Orbs) are the souls of those who cannot find their way into the afterlife. They have no unfinished business or desire to remain, they are simply lost. Due to their lack of purpose, Wisps are MUCH weaker than other types of spirts, and normally cannot even be seen by the naked eye. They manifest as floating orbs of shimmering light. As one might expect, they are completely harmless."

Phantoms

Type: Supernatural

Threat Level: Low

"Phantoms are the spirits of those who have died but have unfinished business in the living world. It could be they were murdered and need to be avenged to pass on, it could be they never confessed feelings for a romantic interest before dying, it could even be because they forgot to share important information. Whatever the case may be, the Phantom will linger in the land of the living until their business has been finished. Phantoms are not particularly lethal spirits, with most deaths involving a Phantom being related to either heart attacks caused by the fright from witnessing one or the unfortunate resemblance between the victim and someone who wronged the Phantom in life. The form taken by a Phantom reflects it's non-aggressive nature, as Phantoms retain their mortal appearance with the exception of being transparent and glowing with a bluish green hue."

Poltergeists

Type: Preternatural

Threat Level: Medium 

"Poltergeists are psychical manifestations of negative emotions, such are Anxiety, Stress, Anger, or Sadness. Poltergeists love nothing more than to cause chaos and panic in the psychical world. They delight in scaring their victims by tampering with electronics, throwing objects, slamming doors, cabinets, and drawers, cackling like madmen, pushing, shoving, and tripping people, and otherwise being a menace. Although poltergeists enjoy the suffering of others, their antics are more along the lines of impish pranks than active malevolence. The only deaths that poltergeists cause are the result of the specters overestimating the durability of their victim's squishy mortal bodies. Poltergeists will also never harm animals or children. In terms of their appearance, poltergeists will often chose to manifest as strange, unsettling, and unearthly creatures of all shapes and sizes."

Wraiths

Type: Supernatural

Threat Level: High

"Wraiths are, to put it simply, the lingering souls of those who lived a life of malice and evil. Wraiths often remain in the living world by choice, wishing to spend their days causing terror and suffering rather than passing on. They take great delight in torturing and killing their victims, and are the most lethal of the supernatural-class spirits. In terms of appearance, expect Wraiths to take a humanlike form, but a warped and horrifying one."

Demons

Type: Preternatural

Threat Level: Lethal 

"Demons are hands down the deadliest and most terrifying spirits one can encounter. They are summoned into the mortal world via dark and depraved rituals by those who foolishly believe they can be controlled. Their ignorance always leads to their demise. Demons love to terrify and torture their prey both mentally and psychically, often putting in a great deal of effort to do so. They often choose a single target to terrorize until they've driven them completely insane. When the victim has been drained of sanity the demon will consume their soul and move on to a new house. Demons uses every method available to them to traumatize their prey (which, considering the terrifying amount of power a demon wields, is quite a lot of methods). They have been known to violently assault their victims, move, smash, and stack objects, scratch threatening messages into walls, lower room temperatures, start fires, cause vivid night terrors and hallucinations, and even violently attack and sometimes kill friends, loved ones, and pets of the victims in front of them. A demon's haunting cannot be stopped by moving houses, as the demon will follow it's prey, latched on like a parasite. A demon can take a wide variety of forms, but will most often appear as some sort of terrifying beast or eldritch horror."